Rheological additives, generally referred to as thickeners, thixotropes or rheological control agents, have long been used in painting compositions for various purposes. Thus, additives such as viscosity control agents, storage-stability agents, anti-settling agents, sag-control agents, and other materials are added to non-aqueous paint and coating compositions in minor amounts. Rheology additives or modifiers (also often called thickeners, rheological control additives and modification thixotropes) are added to such compositions not only to alter the viscosity of the coating composition, but also to maintain the viscosity at desired levels under varying process conditions and end-use situations. Other effects obtained from rheology modifiers include improvement in pigment suspension, leveling and flow. Some of these properties are also desired in other types of compositions, for instance, oil well drilling fluids. For purpose of this invention, chemical compositions providing one or more of the above properties are referred to as rheological additives, and the properties imparted are all included within the term thixotropy.
Such rheological control agents, to be effective, especially for paints and coatings, must provide proper control characteristics to the systems in which they are employed.
For example, organophilic clays have been shown to be useful to thicken various organic and solvent-based compositions. Among numerous United States patents owned by Rheox, Inc., the assignee of this invention, several describe various kinds of organically - modified clays as rheological additives--see U.S. Pat. Nos. 4,208,218; 4,412,018; 4,517,112; 4,695,402; and 5,075,033. Fumed silica and precipitated silica have also been used to thicken certain types of organic systems.
Organically modified clays and silicaceous materials present drawbacks, however, for thickening non-aqueous compositions. Both organically modified clays and fumed silica exist in solid particulate, or powder form, and these materials generally must be added as solids during the grind stage of manufacture of the compositions to be thickened. Organoclays have sometimes been sold as liquid gels, where the organoclay is dispersed into an organic liquid, in order to avoid the disadvantages of using a solid thickener, but such products have limited acceptance in the marketplace.
The use of these types of additives can also lead to a loss of gloss and quality in the final paint or coating. Furthermore, these additives cause the systems in which they are incorporated to exhibit extremely rapid recovery following shear, thereby limiting the leveling or smoothness of the applied composition.
Some of the problems of use and dispersibility associated with the solid or powder types of such thickeners as organoclays are overcome by the use of polyamide rheological additives. For example, Rheox, Inc. U.S. Pat. No. 4,778,843 describes a solid polyamide rheological additive especially designed for organic solvent-based compositions, which comprises the reaction product of polycarboxylic acid, an active hydrogen compound of a specified carbon chain length and structure, and a monocarboxylic acid capping agent. Recent Rheox U.S. Pat. No. 5,349,011 describes a polyamide-ester rheological additive, especially for organic, solvent-based compositions, which comprises the reaction product of polycarboxylic acid, an active hydrogen composition of a specified structure, an alkoxylated polyol, and a monocarboxylic acid capping agent. Said additive is described as exhibiting excellent efficiency and ease of dispersibility when used in aliphatic solvent-based coating compositions, and as effective when dispersed into a solvent.
U.S. Pat. No. 4,337,184 describes a rheology modifier useful in water-based and organic solvent-based compositions derived from the reaction of polyalkylene oxide, polyfunctional material which includes polyols, amines, amine alcohols, thiols and polyisocyanates, including diisocyanates and water. The modifiers are characterized by having a branched structure and containing substantially no terminal hydrophobic moities.
While not a rheological additive, U.S. Pat. No. 4,072,641 describes polyamide resins useful as flexographic ink vehicles which are prepared by reacting polymeric fatty acids, an alkylene diamine, a mono amino alcohol which is neither branched nor ethoxylated, and a chain-stopping agent, which agent includes a particular branched chain monocarboxylic fatty acid. U.S. Pat. No. 5,100,438 describes an ester-amide additive useful for coal-water slurries which is obtained by the reaction of a polycarboxylic acid with a polyether glycol and an aliphatic amine. Salts of these ester-amides are also disclosed. The resulting materials are combined with water, and the water in turn is mixed with the coal in a mixer. The resulting slurries are liquids at ambient temperature.
Rheox U.S. Pat. No. 4,499,233 describes a water-dispersible modified polyurethane polymer as a viscosity increasing composition for aqueous systems. The polymer is discussed as the reaction product of a polyisocyanate, a polyether polyol in a defined molar range, a modifying agent, and a capping agent reactive with the reactive product of the polyisocyanate, the polyether polyol and the modifying agent. Capping agents described include mercaptans, primary and secondary amines and monoisocyanates.
Recent U.S. Pat. No. 5,319,055 shows a thickening agent for thickening solvent-containing compositions described as the reaction product of a polyol containing at least two hydroxyl groups, a polyisocyanate containing at least two isocyanato - groups and an active hydrogen compound having the formula R-X wherein X is selected from the group consisting of primary amino, secondary amino, and hydroxyl, and R represents a group comprising from 1 to 30 carbon atoms. All the active hydrogen compounds shown in the examples are mono-functional except for examples 5 and 15 which have dual functionality. The thickening agents produced are dispersed into solution with toluene prior to being used as thickeners for non-aqueous dispersions.
Two recent patents issued to Rheox, Inc., describe liquid pourable rheological additives based on two types of alkoxylated nitrogen-containing chemistry--see U.S. Pat. Nos. 5,536,871 and 5,510,452, the teaching of which are incorporated by reference herein.
The aforesaid thickeners are in the main, solid products, unless mixed with solvents.